1. Field Of The Invention
This invention relates to semiconductor circuits and, more particularly, to methods and apparatus for providing a precision voltage reference utilizing a pair of flash EEPROM memory cells.
2. History Of The Prior Art
Precision voltage references are well known in the prior art. There have been many circuit arrangements for providing such references. However, as circuits become more and more specialized, it becomes necessary to produce such voltage references using the processes available for producing the associated circuitry. For example, the use of voltage references with large scale integrated circuits may require that the circuitry for producing the reference voltages be manufactured by the processes used for manufacturing the large scale integrated circuits. Thus, it may be possible to provide a precision voltage reference easily where individual components including precision resistors and capacitors are available. However, where integrated circuit devices must be used and both the currents and voltage which may be used by such devices are limited, the ability to provide such references becomes much more difficult. Consequently, the components available for providing such voltage references become more and more limited as the type of integrated circuits becomes more specialized and as the individual elements of the circuitry become smaller and the area more restricted.
One type of integrated circuit which utilizes precise voltage levels is a memory array. Normally, precision voltages are available from external circuitry for interrogating a memory array during a read process or for writing to such an array. However, newer flash EEPROM memory arrays used for long term storage in portable computers are designed to operate with microprocessors which use lower source voltages and do not have precision power supplies available to provide the precise higher voltage values required for programming and erasing such flash arrays. Consequently, it is necessary to generate such precision voltage references within the integrated circuit which contains the array or its control circuitry. This requires that the elements needed for providing the precision voltage references be produced by the process used in manufacturing the flash memory array.